As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-35%.
In many applications, it is desirable to provide a closure or cap for mating with a finish of a container. Many such container and cap combinations are designed with a tamper-evidence (TE) breakaway band on the cap. Such a band is attached to the cap when initially applied to the corresponding container finish and upon opening the container for the first time, the band is designed to break away from the cap and remain on the container. Since the band can only break away one time, the resulting effect proves whether or not the container has been tampered with, or more specifically, if the cap has been removed prior to the actual end user opening the container.
In addition, an improved blown definition may be achieved around a finish of the blown finish type having a debossed (grooved) threaded profile as compared to a conventional embossed (raised) threaded profile. Explained further, during the forming of a PET container with a blown finish, tighter, more functional radii may be created when the material is blown against more defined mold features (i.e. debossed threaded profile) versus blowing the material against milled out mold features (i.e. embossed threaded profile).
Within the realms of the PET blow molding industry, where it is desirable to convert injection molded PET preforms into blow molded PET containers, it has been shown that any blow moldable detail that is designed and built into any given blow mold, be sized in such a way that ensures duplication of that mold detail onto the moldable surface of the resultant container. The inherent nature of PET causes the molded container to become stiffer as it biaxially orientates. As a result, it is important to define any embossed detail as having a height dimension (i.e. in a direction along the axis of the container) to be sufficiently greater than a depth dimension (i.e. in a direction generally transverse to the axis of the container).